1. Field of the Invention
The present invention relates to power supply circuits, and particularly to a circuit including a first power supply line continuously supplied with power and a second power supply line supplied with power only when a switch between a power supply and itself is closed.
2. Related Art
Power supply circuits that are associated with electronic control units (ECUs) for performing various types of control functions, such as vehicle engine control, are well known. Specifically, as shown in FIG. 6, such a power supply circuit 100 includes a first power supply line BATT connected to a power supply terminal T1. The terminal T1 in turn is connected to a battery power supply BT through an ECU connector C, and is continuously supplied with power from the battery BT. Further, a second power supply line +B is connected to a power supply terminal T2 and a ground line G connected to ground terminals T3, T4. The second power supply line +B is supplied with power only when the contact of a relay RLY provided between the battery BT and the terminal T2 is closed. The relay RLY is configured such that the contact is opened and closed in accordance with the operation of an ignition switch IG.
The first power supply line BATT is used to supply power to apparatuses and circuits which require a continuous source of power regardless of the operation of the ignition switch IG, whereas the second power supply line +B is used to supply power to apparatuses and circuits which are activated in response to an ON state of the ignition switch IG.
In the power supply circuit 100, constant voltage circuits 10, 20 are connected to the first power supply line BATT and the second power supply line +B, respectively. The constant voltage circuit 10 connected to the first power supply line BATT is formed by a resistor 12 connected to the first power supply line BATT at one end and to an output terminal of the constant voltage circuit 10 at the other end, and a Zener diode 14 connected to the circuit output terminal at the cathode and to the ground line G at the anode. The output of the constant voltage circuit 10 is normally used as a back-up power supply for maintaining the contents of a data storage circuit such as a RAM and, therefore, the voltage output by the circuit must only be within a predetermined allowable range required for the RAM or the like to normally operate.
The constant voltage circuit 20 connected to the second power supply line +B comprises a PNP type power transistor 22 connected to the second power supply line +B at the emitter and to an output terminal of the constant voltage circuit 20 at the collector, and a well-known constant voltage control IC (e.g., TA7900 manufactured by TOSHIBA) 24. The IC includes an enable terminal e connected to the second power supply line +B, a reference power supply terminal f connected to the first power supply line BATT, a monitor terminal m connected to the collector of the transistor 22, and a control terminal b connected to the base of the transistor 22. The IC operates when a voltage applied to the enable terminal e is equal or greater than a preset operational threshold (4 V) to control the base current of the transistor 22 through the control terminal b such that a voltage applied to the monitor terminal m (that is, the output voltage of the constant voltage circuit 20) becomes constant (e.g., 5.0 V), and is configured such that the output voltage of the constant voltage circuit 20 can be kept constant with a high degree of accuracy to enable circuits in microcomputers and the like to be accurately controlled.
The constant voltage control IC 24 incorporates a reference voltage generation circuit which utilizes a band gap of a semiconductor to generate a reference voltage having substantially no temperature-related fluctuation in order to generate the required constant voltage with a high degree of accuracy. The IC is supplied with power through the reference power supply terminal f to operate the reference voltage generation circuit. The reference power supply terminal f is connected to the first power supply line BATT to enable the reference voltage to be established at the reference voltage generation circuit when the relay RLY is closed. As a result, the constant voltage control IC 24 can perform accurate control.
The power supply circuit 100 further includes a diode 4 connected between the first and second power supply lines BATT, +B whose forward direction is the direction in which a current flows from the second power supply line +B to the first power supply line BATT, and a capacitor 6 connected between the second power supply line +B and the ground line G.
Countermeasures exist to prevent instantaneous interruption on the first and second power supply lines BATT, +B caused by poor contact at connector terminals T1-T4, chattering at the relay contact and the like attributable to vehicle vibration during the operation of the engine and the like. For example, when the supply of power from the battery BT to the first power supply line BATT is interrupted, the supply of power to the first power supply line BATT is maintained with power supplied from the second power supply line +B through the diode 4. When the supply of power from the battery BT to the power supply line +B is interrupted, the capacitor 6 is discharged to continue to provide power to the second power supply line +B. The connector terminals T3, T4 are duplicated to cope with such instantaneous interruption of the ground line G.
In order to improve the kinetic and exhaust gas cleaning performance of vehicles, an increased number of apparatuses and circuits are being connected to vehicle ECUs. Normally, power must be supplied to such apparatuses and circuits through an additional output of the constant voltage circuit 20 via a third power supply line DL that exhibits a stable voltage. However, the need for such an additional circuit increases the amount of power supplied through the second power supply line +B which supplies power to the constant voltage circuit 20.
When the amount of power supplied through the second power supply line +B is increased, a capacitor having a larger capacity is also required to cope with instantaneous interruption as described above. However, the larger capacitor increases the size of the power supply circuit.
In addition, an aluminum electrolytic capacitor commonly used as a high capacity capacitor is subjected to significant changes in its characteristics related to temperature in a high temperature automotive environment, and its capacity is gradually reduced. As a result, the reliability of the power supply circuit is compromised in its operation in, for example, reducing the duration of instantaneous power line interruption that can be coped with, as well as reducing the operational life of the circuit.
Further, while poor contact at the connector terminals may be overcome by duplicating the connector terminals T1, T2 in a manner similar to the ground line G, the size of the connector is increased as are the number of wire harnesses. As a result, the size of the apparatus and the complexity of the associated wiring are increased.